Oral squamous cell carcinoma (SCC) is a debilitating and deadly illness. Despite advances in conventional therapy, oral cancer continues to have unacceptable morbidity and mortality. Given the poor prognosis associated with existing therapies, the Principal Investigator's studies will focus on developing more effective treatments premised on reversing the apoptotic and proliferative defects in SCC. The INK4a/ARF gene locus represents the second most frequently altered gene in human cancer and is altered in 80% of SCC, suggesting its importance in the pathogenesis of this tumor type. Utilizing alternative reading frames, the mammalian ARF-INF4a locus encodes two unrelated proteins that both function in tumor suppression. p16INK4a maintains the retinoblastoma protein in its growth suppressive state through inhibition of cyclin D-dependent kinase activity, while ARF binds with MDM2 and blocks MDM2 and p53 nuclear export, and thus cytoplasmic degradation of p53. These findings implicate both INK4A and ARF in suppressing the development of SCC. Previously, the Principal Investigator and his colleagues (Zhang et al., 1999a) and others (Pomerantz et al., 1998) found that ARF binds the MDM2 oncoprotein leading to stabilization and transcriptional activation of p53 with a resultant proliferative arrest. Recently, the Principal Investigator and coworkers have found that many cancer-derived mutations in human ARF exon 2 disrupt its normal nucleolar localization and impair its ability to block nuclear export of MDM2 and P53 (Zhang et al., 1999b) providing a molecular mechanism underlying ARF-mediated p53 stabilization and activation and underscoring the function of ARF in tumor suppression. The preliminary results presented in this proposal identify two previously unrecognized functions of ARF: that ARF induces an S-phase arrest independent of p53 and that the apoptotic response to ARF can be antagonized by functional retinoblastoma (Rb) protein. The later finding suggests the possibility that ARF may selectively target cells with altered Rb function for apoptotic cell death while sparing normal cells. The goals of this proposal are to further define ARF induced apoptosis, p53 independent functions of ARF, and to determine the possible utility of ARF gene therapy for the treatment of oral SCC.